Molybdenum disulfide has long been known as a desirable additive for use in lubricating oil compositions. However, one of its major detriments is its lack of oil solubility. Molybdenum disulfide is ordinarily finely ground and then dispersed in the lubricating oil composition to impart friction modifying and antiwear properties. Finely ground molybdenum disulfide is not an effective oxidation inhibitor in lubricating oils.
As an alternative to finely grinding the molybdenum disulfide, a number of different approaches involving preparing salts of molybdenum compounds have been tried.
In U.S. Pat. Nos. 4,263,152 and 4,272,387, both filed June 28, 1979, and of common inventive entity and assignee to this application, there is a teaching of a class of oil soluble sulfur containing molybdenum complexes prepared by reacting an acidic molybdenum compound, a basic nitrogen composition and a sulfur compound in the presence or absence of a polar promoter, respectively, to form molybdenum and sulfur containing complexes which are reported therein as useful for inhibiting oxidation, imparting antiwear and extreme pressure properties, and/or modifying the friction properties of a lubricating oil. It has now been discovered that lubricating oils are more effectively stabilized against oxidation when said complexes are used in combination with an aromatic amine compound.